The present invention relates to a magnetic recording medium such as a video tape, an audio tape or a computer tape.
At present, thermoplastic resins such as a vinyl chloride and vinyl acetate resin, a vinyl chloride and vinylidene chloride resin, a cellulose resin, an acetal resin, a urethane resin, or an acrylonitrile butadiene resin as described in, for example, U.S. Pat. Nos. 3,634,137, 4,238,548 and 4,307,154 have been used alone or in combination as a binder for commonly used magnetic recording media. However, magnetic recording layers containing these binders are not necessarily satisfactory since the wear resistance of the magnetic recording layers is low and the tape path of the magnetic tapes tends to be easily stained.
In conventional magnetic recording media, thermosetting resins such as a melamine resin or an urea resin have been used, and binders which are cross-linked by a chemical reaction such as an isocyanate compound or an epoxy compound have been added to the above-described thermosetting resins as described in, for example, U.S. Pat. Nos. 4,049,871, 4,154,895 and 4,333,988. However, with the above-described cross-linkable binders, storage stability of the resin solution in which magnetic particles are dispersed is generally poor. That is, pot life is short, physical properties of a magnetic coating composition can not be maintained homogeneous, homogeneous properties of magnetic tapes can not be maintained, a heat treatment step is required to cure the coated layer after coating a magnetic coating composition and drying it thereby requiring a long period of time for manufacturing the magnetic recording medium.
In order to eliminate the above defects, a method has been proposed in which a magnetic recording medium is prepared by using an oligomer and a monomer of an acrylate type as binders and the binder is hardened with electron beam radiation after drying, as disclosed in Japanese Patent Publication No. 12423/72, Japanese Patent Publication (Unexamined) Nos. 13639/72, 15104/72 and 77433/75. Also, a method has been proposed where thermoplastic resins and resins curable with electron beam radiation, and U.S. Pat. Nos. 3,871,908, 4,004,997 and 4,343,861 are used in combination as binders as disclosed in Japanese Patent Publication (Unexamined) Nos. 25231/81, 86130/82, 86131/82, 127926/82 and 162125/82.
In recent years, a support for magnetic layers has been made thinner for long time recording and, thus, the physical strength of the magnetic layer needed to be improved. In addition, video cassette recorders have come to have multi-functions, and video recording tapes are required to withstand use under severe conditions. However, it has been impossible to prepare a magnetic recording medium having high electromagnetic properties and running durability in accordance with the above described prior arts. That is, with conventional binders, the dispersion of ferromagnetic particles has poor stability which, in turn, deteriorates electromagnetic properties as well as running durability of the magnetic recording medium prepared from the dispersion.